Apparatus for crucible-free zone melting of crystalline rods, particularly of semiconductor rods



Dec. 13, 1966 L. MlCHL ETAL 3,291,570

APPARATUS FOR CRUCIBLE-FREE ZONE MELTING OF CRYSTALLINE RODS,PARTICULARLY OF SEMICONDUCTOR RODS Filed Oct. 22, 1963 United Statesatent APPARATUS FOR CRUCIBLE-FREE ZONE MELT- ING F CRYSTALLHNE RODS,PARTICULARLY 0F SEMICONDUCTOR RODS Ludwig Michl, Miesbach, and WernerSpielmann, Dachtel, Kreis Calw, Germany, assiguors to Siemens & HalskeAktiengesellschaft, Berlin, Germany, a corporation of Germany Filed Oct.22, 1963, Ser. No. 318,011 6 Claims. (Cl. 23273) Our invention relatesto an apparatus for carrying out crucible-free zone or floating-zonemelting, and more particularly zone melting of rods consisting ofsemiconductor material.

It is expedient for a series of tasks that are accomplished by usingcrucible-free zone melting, that the heat source which produces themelting zone is stationary and the semiconductor rod that is beingworked is led in the direction of its axis past the heat source, whichpreferably consists of a .short induction coil surrounding the rodconcentrically at the location of the melting zone that is to beproduced. It is moreover desirable and an object of this invention toprovide a zone-melting apparatus in which the two holders of the rodthat is to be melted also run toward each other synchronously at changein direction of movement of the rod. It is an additional object of ourinvention to provide a zone-melting apparatus of this type in which thespacing of both rod holders relative to each other is controllable asrequired independently of the existing variable travel speed of themelting zone dur ing the zone melting process, as is for examplenecessary with thin drawing processes in Which a thin crystal isproduced out of a thicker rod by means of zone melting.

With the above and other related objects in view, and in accordance withour invention, we provide an apparatus for crucible-free zone melting inwhich, on the one hand, the portions of a vertically held rod, that areseparated from one another by the melting zone, are displaceable withrespect to one another as needed and, on the other hand, the rod isdisplaceable as a unit, the spacing between both of its holdersremaining unchanged, in the direction of its axis with respect to theheat source which produces the melting zone.

According to further aspects of our invention, the apparatus includes apair of holders secured to both rod ends and slidingly displaceablyarranged by means of a rigid support arm on a stationary guide, forexample a guide rail, extending parallel to the rod axis. The armscarrying the rod holders are provided with respective screw threadsextending parallel to the guide, and each threaded=ly engaging acorresponding screw thread (worm) respectively carried by a rotary shaftthat extends parallel to the aforementioned guide, so as to producereciprocation of the respective rod holder in the direction of the rodaxis by rotation of the coordinated rotary shaft. Furthermore, bothrotary shafts coordinated respectively with both rod ends are parallelone to another, or prefer ably coaxially aligned and are eachrespectively coupled with -a motor connected between the respectiverotary shaft and its mounting, the motor making possible a rotation ofthe respective rotary shaft and therewith a recipro' cation of the rodend that is connected with the respective rotary shaft. Both rotaryshafts are coupled to each other in such a way mechanically by a pair ofserially arranged, differential gear mechanisms connected one withanother by an additional rotary shaft which extends parallel to andpreferably coaxial with both of the aforementioned rotary shafts, thatby means of an additional motor connected 'betwen the rotary shaftintermediate to both differential mechanisms and a fixed location, thisrotary shaft can be rotated with respect to its surroundings so as toproduce rotation of the outer bevel gears of both equally-dimensioneddifferentials in the same direction and at the same speed by the factthat the respective outer bevel gears .are rigidly connected to eachother and the respective cooperating rotary shafts are coupled with therod holders, when both of the remaining motors are switched off.

Other features which are considered as characteristic for the inventionare set forth in the appended claims. The invention, both as to itsinstruction and method of operation, together with additional objectsand advantages thereof, will be best understood from the followingdescription of specific embodiments when read in connection with theacompanying drawing in which the sole figure shows an elevational viewpartly in section and partly diagrammatic of a zone melting assemblyconstructed in accordance with the invention.

Referring to the drawing there is shown a pair of vertical guide rails1, 1' which are supported parallel to the axis of a silicon rod 6 thatis to be melted. The melting zone 6a is produced by a fixed inductioncoil 7 which concentrically surrounds the rod and is supplied with ahigh frequency current. The ends of therod 6 are secured with set screwsor the like at the ends of two vertically arranged elongated holders 4,5 which are vacuumtightly and glidingly displaceably guided throughapertures in the walls of a perfectly stationary receiving chamber orrece ver 30 which surrounds the rod. The manner in which the rod holders4, 5 may be guided through the wall can be effected as described inPatent No. 3,160,- 478 to T. Rummel et al. and issued December 8, 1964.

In order to maintain a good vacuum in the receiver 30, it is recommendedfurther to insert the rod holders 4, 5 through antechambers 31 which arealso evacuated to effectively prevent the inlet or entry of air into thereceiver 30 itself. The rod holders 4, 5 suitably consist of heatresistant metal which preferably does not give off any gaseousimpurities due to the heating that is required for the zone melting.VZa-sheet (German) containing about 19% chromium and about 9% nickel isan example of a metal that is suitable for this purpose. The receiver 39and the antechambers 31 are suitably connected during operation to acontinously running vacuum pump (not shown), If desired the receiver 30and the antechambers 31 can also be filled with a protective gas.

The rod holders 4, 5 which lie along extensions of the axis of thesilicon rod 6 that is to be processed, are thus vertically oriented inthe embodiment that is illustrated in the drawing, and are rigidlyconnected at their ends to support arms 2, 3 which extend perpendicularto their longitudinal extension. The arms 2, 3 that are disposedadvantageously parallel to one another directly engage the guide rails1, 1' and are able to move only in the direction of these guide rails,i.e. in the vertical direction of the illustrated embodiment. T 0 effectsuch a movement, the support arms 2, 3 are provided with threaded tapholes 2', 3 whose axes extend parallel to the guide rails 1, 1 andconsequently to the axis of the rod 6. Rotary shafts 18 and 19 arrangedcoaxial to one another extend through the tap holes 2 and 3'respectively, parallel to the guide rails 1, 1'. The shafts 18 and 19extend through externally threaded sleeves or worms 18 and 19'respectively which are securely held thereon by pressfitting splines orkeys, or the like. The worms 18' and 19 correspond to and mesh with thethreads of the holes 3', the length of the worm thread 18', 19' being amultiple of the length of the holes 2', 3. Both thread systems 2', 18 or3', 19' have the same pitch and the same thread. When the rotary shafts18, 19 are rotated in the same rotary direction with the same rotaryspeed about their axes, then the spacing of both arms 2, 3 remainsunchanged. However, both arms are displaced vertically upward ordownward so that the rod 6 experiences a uniform or steady translatorymovement in its longitudinal direction with respect to the heat source7, and the melting zone 6a accordingly creeps further or travels alongthe rod.

Both rotary shafts 18, 19 are at no time rigidly connected to oneanother but are rather coupled by means of a pair of differential gearmechanisms 16, 17 connected one behind the other. Both differential gearmechanisms consist of a core 16a, 17a each of which carries a bevel gear16b, 17b which meshes with the bevel gears 16c, 16d or 17c, 17d that aremounted opposite one another at the sides of both differential gearmechanisms. As shown in the figure, the rotary shaft 18 is connectedwith the core 16a of the upper differential gear mechanism 16, and therotary shaft 19 is connected with the core 17a of the lower differentialgear mechanism 17. The gears 16:: and 28 are carried on a common hollowshaft 18a rotatably mounted in the core 16a of the differentialmechanism 16, the shaft 18 extending through the hollow shaft 18a andbeing secured to the core 16a. A similar arrangement is afforded by thehollow shaft 19a for the differential mechanism 17. The gears 16c and17d on the one hand and the gears 16d and 17c on the other hand are ineach case of similar dimensions. As a matter of fact, in the illustratedembodiment of this application all of the spur gears are of equaldimension. Both of the spur gears 160?, 17c are rigidly fitted on theirsides respectively on a connecting shaft 13 which is shown as coaxial tothe rotary shafts 18, 19 in the illustrated embodiment, but which may bearranged parallel to them, and is locatedbetween both differential gears16, 17. The connecting shaft 13 is, however, neither rigidly connectedwith the shafts 18, 19 nor with the cores 16a, 16b of the differentialgears 16, 17.

In the embodiment illustrated in the figure, an additional plate-shapedgear 20 is mounted on the rotary hollow shaft 18a surrounding the shaft18, an additional plate-shaped gear 21 is mounted on the rotary hollowshaft 19a surrounding the shaft 19, and an additional plate-shaped gear22 is mounted on the shaft 13, all rigidly connected to their respectiveshafts. All of these gears 20, 21, 22 mesh respectively with similargears 23, 25, 24 on separately rotatable shafts 10, 12, 11 that aremounted in bearings located on the gear housing 14, these shafts 10, 12,11 also respectively carrying a worm gear 26, 27, 9 driven by respectivemotors 28, 29, 8. A consequence of this arrangement is that the shaft 13and therewith the gears 16d, 170 are rotated when the motor '8 rotates.Also, as a result of the rotation of motor 28-,

the shaft 18 and the bevel gear 160 of the differential gear mechanism16 is set in motion, and by the rotation of motor 29, the shaft 19 andthe bevel gear 17d of the differential gear mechanism 17 are set inrotation.

If the motor 8 is alone in operation, then the cores 16a, 17a of thedifferential gears 16, 17 are not rotated by the rotation of both bevelgears 16d, 17c. The spur gears 16c, 17d nevertheless experience anopposing rotation and indeed witht the same speed of rotation as bothspur gears 16d and 17c when the spur gears 16c, 16d, 17c, 17d are of thesame dimensions. The shafts 18, 19 are then rotated with the same speedas the shaft 13, although in the opposite rotary direction. As a result,both shafts 18, 19 behave like a single rigid shaft and causedisplacement of the rod 6 as a unit relative to the heat source 7,whereby the spacing between both rod holders 4, remains unchanged.

On the other hand, if the motor 28 only is in operation, by means ofwhich the upper shaft 18 is driven, the core 16a of the upperdifferential gear 16 rotates at half the speed as the shaft 18 in thesame direction. The gear 16d, the shaft 13 and the lower differential 17on the contrary remains at rest. Consequently the lower portion of therod 6 does not move while the upper portion of the rod 6 is eitherbrought closer to or drawn away 1 from the lower rod portion and theheat source 7 which produces the melting zone. In a similar manner,actuation of the motor 29 only, causes displacement of the lower rodportion while the upper rod portion remains at rest.

Simultaneous actuation of all three motors 8, 28, 29 causes both axialdisplacement of the rod 6 as a unit toward or away from the heat source7 as Well as also an approach toward one another or a separation fromone another, respectively, of both ends of the rod and therewith acompression or elongation of the melting zone 6a.

The speed of the individual motion components is arrived at byselectively proportioning the ratio of the differential gears, bychoosing a suitable rotary speed of the motors 8, 28, 29, and byappropriate design or arrangement of the screw thread systems 2', 18',or 3, 19'. These screw thread systems are suitably coordinated with oradjusted to one another so that they produce the same displacement speedof the arms 2, 3 with the same rotary speed of the shafts 18, 19, or, inother words, the screw thread systems have the same pitch or range. Thescrew thread systems are interchangeable in suitable manner in that thearms 2, 3 which are formed with the holes 2', 3' respectively and theworm screws 18' and 19' mounted on the rotary shafts 18, 19,respectively, are interchangeable. In order to achieve a simple means ofregulating the rotary speed of the shafts 13, 18 and 19, it is advisablethat the motors 8, 28, 29 be direct-current motors. With motors of thistype the speed and the direction of rotation can be regulated in asimple manner by the voltage that is fed to the field coils and therotor as well as by the choice of polarity of these voltages. It is thenpossible to switch the motors and thereby the rotary speed of the rotaryshaft continuously from a maximum speed in one direction to a maximumspeed in the other direction.

While the invention has been illustrated and described as embodied in aparticular apparatus for crucible-free zone melting of crystalline rods,it is not intended to be limited to the details shown, since variousmodifications and structural changes may be made without departing fromthe spirit of the present invention and within the scope and range ofequivalents'of the following claims.

We claim:

1. An apparatus for effecting crucible-free zone melting of asemiconductor rod, comprising guide means extending parallel to the axisof the rod; rod-holder means secured to opposite end portions of the rodand slidahle along said guide means; heating means for melting a zone ofthe rod intermediate the end portions of the rod; and means for slidingsaid rod-holder means and consequently the rod end portions along saidguide means, said sliding means comprising a pair of independentlyrotatable first and second shaft screw thread means for respectivelydisplacing each of said rod-holder means and the respective rod endportion, a differential mechanism operatively connected to each of saidfirst and second shaft screw thread means, rotary shaft means coaxialwith said first and second shaft screw thread means serially couplingsaid differential mechanisms to each other, first and second motor meansfor independently rotating said first and second shaft screw threadmeans so as to selectively displace one of said rod-holder means and therespective rod end portion relative to the other, and third motor meansfor imparting rotation to said rotary shaft means and, through saiddifferential mechanisms, to said first and second shaft screw threadmeans independently of said first and second motor means so as tosimultaneously displace both of said rod-holder means and respective rodend portions in the same direction and at the same rate of speed.

2. An apparatus for effecting crucible-free zone melting of asemiconductor rod, comprising a pair of guide rails extending parallelto the axis of the rod; a pair of spaced support arms extendingtransversely to said rails and slidably mounted thereon; a .pair ofrod-holders secured respectively to opposite end portions of the rod andfixed respectively to said support arms for sliding therewith; heatingmeans for melting a zone of the rod intermediate the end portions of therod; and means for sliding said support arms and consequently the rodend portions along said guide rails, said sliding means comprising apair of independently rotatable first and second shaft screw threadmeans for respectively displacing each of said support arms and therespective rod end portion, a differential mechanism operativelyconnected to each of said first and second shaft screw thread means,rotary shaft means serially coaxial with said first and second shaftscrew thread means coupling said differential mechanisms to each other,first and second motor mean for independently rotating said first andsecond shaft screw thread means so as to selectively displace one ofsaid support arms and the respective rod end portion relative to theother, and third motor means for imparting rotation to said rotary shaftmeans and, through said differential mechanisms, to said first andsecond shaft screw thread means independently of said first and secondmotor means so as to simultaneously displace both of said support armsand respective rod end portions in the same direction and at the samerate of speed.

3. An apparatus for effecting crucible-free zone melting of asemiconductor rod, comprising a pair of guide rails extending parallelto the axis of the rod; a pair of spaced support arms extendingtransversely to said rails and slidably mounted thereon; a pair ofrod-holders secured respectively to opposite end portions of the rod andfixed respectively to said support arms for sliding movement therewith;heating means for melting a zone of the rod intermediate the endportions of the rod; and means for sliding said support arms andconsequently the rod end portions along said guide rails, said slidingmeans comprising a pair of similar threaded bores formed in said supportarm and having an axis parallel to said guide rails, and a pair ofindependently rotatable shafts each having an externally threadedportion cooperating with the respective threaded bores for selectivelydisplacing each of said support arms and the respective rod end portion,a differential mechanism operatively connected to each of saidexternally threaded shafts, rotary shaft means coaxial with theexternally threaded portions of said pair of shafts serially couplingsaid differential mechanisms to each other, first and second motor meansfor independently rotating said externally threaded shafts in saidthreaded bores so as to selectively displace one of said support armsand the respective rod end portion relative to the other, and thirdmotor means for imparting rotation to said rotary shaft means andthrough said differential mechanisms, to said externally threaded shaftsindependently of said first and second motor means so as tosimultaneously displace both of said support arms and respective rod endportions in the same direction and at the same rate of speed.

4. An apparatus according to claim 3 wherein said guide rails arevertically disposed, and said rod-holders are rod-shaped and extendcoaxial with the rod.

5. An apparatus for efiecting crucible-free zone melting of asemiconductor rod, comprising a pair of guide rails extending parallelto the axis of the rod; a pair of spaced support arms extendingtransversely to said rails and slidably mounted thereon; a pair ofrod-holders secured respectively to opposite end portions of the rod andfixed respectively to said support arms for sliding therewith; heatingmeans for melting a zone of the rod intermediate the end portions of therod; a receiving chamber in which the rod is heated, said chamber havingevacuated antechambers at either end through which said rod-holder meansare slidingly reciprocable; and means for sliding said support armsalong said guide rails, said sliding means comprising a pair ofindependently rotatable first and second shaft screw thread means forrespectively displacing each of said support arms and the respective rodend portion, a differential mechanism operatively connected to each ofsaid first and second shaft screw thread means, rotary shaft meanscoaxial with said first and second shaft screw thread means seriallycoupling said differential mechanisms to each other, first and secondmotor means for independently rotating said first and second shaft screwthread means so as to selectively displace one of said support arms andthe respective rod end portion relative to the other, and third motormeans for imparting rotation to said rotary shaft means and, throughsaid differential mechanisms, to said first and second shaft screwthread means independently of said first and second motor means so as tosimultaneously displace both of said support arms and respective rod endportions in the same direction and at the same rate of speed.

6. An apparatus for effecting crucible-free zone melting of asemiconductor rod, comprising guide means extending parallel to the axisof the rod; rod-holder means secured to opposite end portions of the rodand slidable along said guide means; heating means for melting a zone ofthe rod intermediate the end portions of the rod; and means for slidingsaid rod-holder means and consequently the rod end portions along saidguide means, said sliding means comprising a pair of independentlyrotatable first and second shaft screw thread means for respectivelydisplacing each of said rod-holder means and the respective rod endportion, a differential mechanism op eratively connected to each of saidfirst and second shaft screw thread means, rotary shaft means coaxialwith said first and second shaft screw thread means serially couplingsaid differential mechanisms to each other, first and second motor meansfor independently rotating said first and second shaft screw threadmeans so as to selectively displace one of said rod-holder means and therespective rod end portion relative to the other, and third motor meansfor imparting rotation to said rotary shaft means and, through saiddifferential mechanisms, to said first and second shaft screw threadmeans independently of said first and second motor means so as tosimultaneously displace both of said rod-holder means and respective rodend portions in the same direction and atthe same rate of speed, each ofsaid motor means comprising a direct current motor, the rotary speed androtary direction of said motor being controllable by varying theenergizing voltage.

References Cited by the Examiner UNITED STATES PATENTS 2,389,047 11/1945Reinz 74-675 X 2,493,828 1/1950 Patsons 74675 X 3,160,478 12/1964 Rummelet al 23-301 X NORMAN YUDKOFF, Primary Examiner.

ROBERT F. BURNETT, Examiner.

G. P. HINES, Assistant Examiner.

1. AN APPARATUS FOR EFFECTING CRUCIBLE-FREE ZONE MELTING OF ASEMICONDUCTOR ROD, COMPRISING GUIDE MEANS EXTENDING PARALLEL TO THE AXISOF THE ROD; ROD-HOLDER MEANS SECURED TO OPPOSITE END PORTIONS OF THE RODAND SLIDABLE ALONG SAID GUIDE MEANS; HEATING MEANS FOR MELTING A ZONE OFTHE ROD INTERMEDIATE THE END PORTIONS OF THE ROD; AND MEANS FOR SLIDINGSAID ROD-HOLDER MEANS AND CONSEQUENTLY THE ROD END PORTIONS ALONG SAIDGUIDE MEANS, SAID SLIDING MEANS COMPRISING A PAIR OF INDEPENDENTLYROTATABLE FIRST AND SECOND SHAFT SCREW THREAD MEANS FOR RESPECTIVELYDISPLACING EACH OF SAID ROD-HOLDER MEANS AND THE RESPECTIVE ROD ENDPORTION, A DIFFERENTIAL MECHANISM OPERATIVELY CONNECTED TO EACH OF SAIDFIRST AND SECOND SHAFT SCREW THREAD MEANS, ROTARY SHAFT MEANS COAXIALWITH SAID FIRST AND SECOND SHAFT SCREW THREAD MEANS SERIALLY COUPLINGSAID DIFFERENTIAL MECHANISMS TO EACH OTHER, FIRST AND SECOND MOTOR MEANSFOR INDEPENDENTLY ROTATING SAID FIRST AND SECOND SHAFT SCREW THREADMEANS SO AS TO SELECTIVELY DISPLACE ONE OF SAID ROD-HOLDER MEANS AND THERESPCTIVE ROD END PORTION RELATIVE TO THE OTHER, AND THIRD MOTOR MEANSFOR IMPARTING ROTATION TO SAID ROTARY SHAFT MEANS AND, THROUGH SAIDDIFFERENTIAL MECHANISMS, TO SAID FIRST AND SECOND SHAFT SCREW THREADMEANS INDEPENDENTLY OF SAID FIRST AND SECOND MOTOR MEANS SO AS TOSIMULTANEOUSLY DISPLACE BOTH OF SAID ROD-HOLDER MEANS AND RESPECTIVE RODEND PORIONS IN THE SAME DIRECTION AND AT THE SAME RATE OF SPEED.